1. Field of the Invention
The present invention relates to an X-ray tube apparatus of a rotating anode type and, in particular, an improvement in the structure of a rotating anode type X-ray tube as a vacuum container equipped with a metal container section for receiving an anode target, in the structure of an X-ray tube holding housing for holding the rotating anode type X-ray tube and in the structure of a stator for rotational drive.
2. Description of the Related Art
As is well-known in the prior art, the rotating anode type X-ray tube is mounted within an X-ray tube holding housing filled with an insulating oil. The X-ray tube apparatus of a rotating anode type is equipped with a stator of an electromagnetic induction motor for rotating the X-ray tube at high speeds. The stator above is comprised of an iron core/coil conductor-combined unit and located near the outer periphery of a vacuum envelope for housing the rotary structure in the X-ray tube corresponding to a rotor of the motor.
As shown in FIG. 1, the stator 13 is constructed by a stator coil conductor 12 wound along a number of slits formed in a cylindrical iron core 11, that is, a core comprised of stacked thin sheet rings made of a ferromagnetic material. On the other hand, the X-ray tube 14 is equipped, with a glass container section 17 of a vacuum envelope 16 surrounding a rotary structure 15. A disc-like anode target 19 is arranged in the vacuum envelope 16 at a metal container section 18 of a large diameter. The anode target 19 is fixed by a rotation shaft 20 to the rotary structure 15 and supported there. The rotary structure 15 is rotatably held on a stationary structure 21 by bearing means not shown. In FIG. 1, reference numeral 18a denotes a corona ring extending from the metal container section; 17a, an expanding flared section of the glass container section; and 17b, a small-diameter cylindrical section of the glass container section.
The stator 13 is arranged near the outer periphery of the small-diameter cylindrical section 17b of the glass container section. A rotation magnetic field is generated mainly on the inside of the iron core 11, acting upon the rotary structure 15 and hence rotating the rotary structure at high speeds.
With the conventional X-ray tube apparatus having a structure as shown in FIG. 1, the coil conductor 12 of the stator 13 linearly extends toward the anode target side and the ion core 11 is relatively spaced far apart from the anode target 19. From the structural and operational condition of the X-ray tube apparatus, usually, the metal container section 18 of the vacuum container (envelope) is held at a ground potential and a high positive voltage of, for example, 75 kV is applied to the anode target 19. For this reason, an interval G between the anode target 19 and the metal container section 18 of the vacuum container is maintained at a distance enough great to withstand such a high voltage difference during operation.
The axial distance H from the lower end of the anode target 19 to that of the rotary structure 15 is 10 increased to an undesired extent. Further, the iron core 11 of the stator 13, together with the X-ray tube holding housing, is connected to a ground potential and the iron core and the coil conductor are substantially connected to the ground D.C. potential, even if an AC drive voltage is applied to a coil conductor 12 at the operation of the X-ray tube apparatus. During the operation of the X-ray tube apparatus, a great potential gradient is involved on the inner surface of the expanding flared section 17a of the glass container section due to a potential distribution created between the inside corner portion of the upper end of the stator 13 and the rotary structure in the X-ray tube. Floating electrons e entering into the space between the corona ring 18a and the rotary structure 15 reach the inner surface of the expanding flared section 17a which is charged up by the floating electrodes. This may develop an undesired discharge.